1. Field of the Invention
The present invention relates generally to motorcycle exhaust devices that can be reduced in body-width to be slim and in particular to a motorcycle exhaust device suitable for off-road vehicles.
2. Description of Background Art
A motorcycle is known in which an exhaust pipe extending from an exhaust port of an engine is bent, passing between a main frame and the engine and extending rearwardly, and is connected to a muffler. See, for example, Japanese Patent Laid-Open No. 2006-321489.
A motorcycle is known in which exhaust pipes extending from a single engine are arranged to bifurcate on the left and right of a vehicle at a position rearward of a shock absorber and are connected to corresponding mufflers on the left and right of a vehicle body. See, for example, Japanese Patent Laid-Open No. 2005-313671.
FIG. 13 is a lateral view illustrating a rear portion of an exhaust system in one of the conventional examples. A muffler 223 is of an inner-outer double-tube type in which an inner pipe 221 made of punching metal is disposed in the longitudinal direction of the muffler 223 and inside an outer tube exhibiting the external appearance of the muffler 223. The inner pipe 221 is connected at a front end to an exhaust pipe (not illustrated in FIG. 13) via a joint pipe 220. In addition, the inner pipe 221 is connected at a rear end to a tail pipe 280. An end cap 254 is attached to the rear end opening of the muffler 223 (the outer tube) to close it. The tail pipe 280 passes through the end cap 254 and communicates with the posterior atmosphere at a rear end opening.
The front end opening of the muffler 223 (the outer tube) is closed by a cone-shaped front cap 270. The inside of the muffler 223 (the outer tube) closed by the front cap 270 and the end cap 254 is formed as an expansion chamber. The exhaust gas inside the inner pipe 221 comes out thereof and goes into the expansion chamber 256 to expand therein. Thereafter, the expanding exhaust gas is squeezed in the tail pipe 280 and discharged into the atmosphere. In this way, exhaust sound pressure is reduced.
However, in such a case where the exhaust pipe is made to pass between the engine and the main frame as in Japanese Patent Laid-Open No. 2006-321489 mentioned above, it is conceivable that a recessed portion is provided in the main frame in order to ensure an arrangement space for the exhaust pipe. However, this leads to the increased size and weight of the entire main frame in order to ensure the rigidity of the main frame. In addition, it is conceivable that the main frame per se is shifted outwardly so as to be away from the engine to ensure a space-width. However, if so, the body-width will be increased. Therefore, it is required to reduce the body-width to make the vehicle body slim.
Making a vehicle-body slim as described above is strongly required particularly in an off-road vehicle operating in the wilderness such as an irregular terrain or the like. It is also required to sufficiently reduce exhaust sound. Consequently, the muffler is increased in size, which may hinder the slimming of the vehicle body in some cases.
On the other hand, to arrange the exhaust pipe in order to achieve the slimming of the vehicle body, it is conceivable that the exhaust pipe intersecting the main frame is partially made concave without modifying the main frame. However, it is necessary to consider the shape of the exhaust pipe additionally in order to make the flow of exhaust gas smooth.